Everything that is singular for friendly nations (6)
I believe the answer is:
allies
'friendly nations' is the definition.
(I've seen this before)
'everything that is singular' is the wordplay.
'everything' becomes 'all' (all things).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'all'+'ie'+'s'='ALLIES'
'for' acts as a link.
(Other definitions for allies that I've seen before include "WW2 victors" , "They join and fight together" , "Nations aligned against Germany in WW2" , "They are banded together for mutual support" , "Co-operating states" .)